In general, GaN-based semiconductors are applied to fields of light devices, such as blue/green light emitting device (LED), high speed switching elements, such as MESFET(Metal Semiconductor Field Effect Transistor), HEMT(High Electron Mobility Transistors) and the like, and electronic devices as high power devices.
In the general types of the GaN-based semiconductor LEDs, the GaN-based semiconductor LED is fabricated by a method including growing a thin polycrystalline film as a buffer layer on a substrate (e.g., sapphire substrate or SiC substrate) at a low growth temperature, forming an n-GaN layer on the buffer layer at a high growth temperature, and forming a magnesium (Mg)-doped p-GaN layer on the buffer layer. An active layer emitting light is sandwiched between the n-GaN layer and the p-GaN layer.
Meanwhile, in the conventional pn-junction LED and fabrication method thereof, crystal defects may be generated due to a difference in lattice constant and a difference in thermal expansion coefficient between the sapphire substrate and the GaN semiconductor. To suppress the occurrence of such crystal defects, a low temperature GaN-based or AlN-based buffer layer is applied, obtaining a GaN semiconductor having a crystal defect size of ˜108/cm1. Hereinafter, the occurrence path of the crystal defect and the conventional method employed to suppress the crystal defect will be described.
In brief, if amorphous GaN-based or AlN-based buffer layer is formed at a low temperature and is then recrystallized at a high temperature, a ‘poly-like’ crystal is formed, which is very rough in surface state and is not good in flatness. However, as the crystal growth continues, a vertical growth is preferrentially performed at a first stage and then two-dimensional growth is preferentially performed at a second stage, so that good quality of nitride semiconductor can be obtained.
At this time, in the vertical growth period corresponding to the initial growth phase, the crystal defect is generated at a boundary fused with a GaN island. The crystal defects are generated in a variety of forms, for example, ‘threading dislocation’, ‘screw dislocation’, ‘line dislocation’, ‘point defect’ that are propagated to a surface of the LED, or ‘mixture’ of the aforementioned defects. Eventually, the crystal defects badly influence the device reliability. In particular, while the ‘threading dislocation’ is propagated to the surface of the LED from the sapphire substrate, it passes through the active layer emitting light. In the future, the ‘threading dislocation’ serves as a current path of leakage current or the like, and accordingly, when a high voltage such as ESD is instantly applied to, the active layer is destroyed or light power is lowered, which serves as a basic reason badly influencing the reliability.
Under this circumstance, to further enhance the light power of the LED and the operation reliability against an external factor such as ESD (electrostatic discharge) or the like, the growth of a GaN semiconductor having less crystal defect is required.
To solve this problem; a variety of growth techniques, such as ‘lateral overgrowth’, ‘pendeo-growth’ or the like using insulator or refractory metal have been employed to decrease the crystal defect to ˜107/cm1 at most. However, the conventional fabrication method has a problem that the process is complicated. Also, although the conventional fabrication method can effectively suppress the crystal defect, it is disadvantageous in terms of costs and still has the need to continue technical development in order to meet the possibility of mass production.
Accordingly, in order to effectively enhance the light power and reliability of the LED, a crystal growth method that can minimize the crystal defect propagated from the substrate is essentially required.